docs: def fitness(self, width, height): 
          """
          Metric used to rate how much space is wasted if a rectangle is placed.
          Returns a value greater or equal to zero, the smaller the value the more 
          'fit' is the rectangle. If the rectangle can't be placed, returns None.
  
          Arguments:
              width (int, float): Rectangle width
              height (int, float): Rectangle height
  
          Returns:
              int, float: Rectangle fitness 
              None: Rectangle can't be placed
          """
          assert(width > 0 and height > 0)
          
          rect, max_rect = self._select_position(width, height)
          if rect is None:
              return None
  
          # Return fitness
          return self._rect_fitness(max_rect, rect.width, rect.height)